A Potentially Habitable World in Our Nearest Star

An international
team of astronomers from the Pale Red Dot
campaign have found evidence of a potentially habitable world orbiting the
closest star to Earth, Proxima Centauri, a cool red-dwarf slightly older
than the Sun [1].
The planet, named Proxima b, has a minimum mass of 1.3 times that of Earth and orbits
its parent star every 11.2 days, receiving about 70% the energy Earth receives
from the Sun.

A potentially habitable world is a planet around another
star that might support liquid water on its surface and therefore lies within the so-called habitable zone. Though currently we cannot tell exactly how
habitable such planets are because we can not investigate their geologic or atmospheric composition, it is believed that small planets located in the habitable zone, just like Earth and now Proxima
b, would be more likely to have the right conditions for life as we
know it.

Proxima b was added
to the Habitable Exoplanets Catalog (HEC)[2]
as one of the best objects of interest for the search for life in the universe. The planet orbits well within the conservative habitable zone of Proxima Centauri. Additionally, Proxima b is now not only the closest potentially habitable planet to Earth (4.2 light
years away), but it is also the most similar to Earth (ESI = 0.87) [3],
with respect to Earth’s mass and insolation.

Other factors, though, make
Proxima b quite different from Earth. It is probably tidally-locked, always
giving the same face to its star. The illuminated side might be too hot, while
the dark side too cold for liquid water or life. A thick atmosphere or a large
ocean, though, could regulate the temperatures across the planet, but we do not know if
this is the case [4].

Probably the most
detrimental factor for the habitability of Proxima b is the activity of its
parent star, which produces strong magnetic fields, flares, and high UV and
X-ray fluxes as most red-dwarf stars do. These factors may lead to the atmospheric
and water loss of the planet, but would not necessarily preclude
habitable conditions [5], [6], [7]. Should Proxima b have a magnetic field, much like Earth does, it would potentially be shielded from such devastating forces.

The mass of Proxima b suggests a rocky composition, but we do
not know its radius to evaluate its bulk density [8].
The planet could be between 0.8 to 1.4 Earth radii depending on composition [9]and if rocky should be about 10% larger than Earth. However, Proxima b could be larger given that we only know its minimum mass.

Statistically, it is not expected to have a potentially
habitable world so close to Earth due to their expected low occurrence in the galaxy.
It is estimated that 24% of red-dwarf stars have an Earth-sized planet (1 to 1.5
RE)
in the optimistic habitable zone [10].
This corresponds to an average separation of eight light years between them in
the Solar Neighborhood (248 red-dwarfs within 10 parsecs) [11].
Therefore, the probability of having a potentially habitable world orbiting our nearest star is less than 10%. Either Proxima b was a lucky find or these
worlds are more common than previously thought.

The most exciting aspect of this discovery is that Proxima b
is relatively close enough to Earth for detailed studies in the next years by
current and future observatories. Other known potentially habitable worlds,
especially those from the NASA Kepler primary
mission, are too far away to get any information about their atmosphere or
composition with current technology. Projects like StarShot are even
considering the possibility of reaching the stars with miniaturized space probes,
but this exciting approach might take many decades.

Proxima b is an excellent object for future characterization
via transit or direct imaging in search for biosignatures [12].
There is a 1.5% chance that Proxima b transits its parent star [13].
Such transits will take 53 minutes as seen from Earth and will produce a notable
0.5% decrease on the brightness of Proxima Centauri [14].
Direct imaging in the next decades might even provide information about the
surface and weather of Proxima b [15].

In any case, Proxima b is now one of the prime targets to
understand the extension of our habitable universe in years to come. Red-dwarf
stars are the most common star in our galaxy, comprising about 75% of the
stars. If we find out that planets around red-dwarf stars, such as Proxima b,
are in fact not habitable then the ‘real estate’ for life in the universe will
be instead very small. The answer lies 4.2 light years away waiting for us.

[2] The
Habitable Exoplanets Catalog (HEC) tracks since 2011 all potentially habitable
worlds discovered by all ground and space telescopes around the world. The HEC is
an initiative of the Planetary Habitability Laboratory (PHL) of the University
of Puerto Rico at Arecibo (UPR Arecibo).

[3]
The Earth
Similarity Index (ESI) is a measure of Earth-likeness from zero (no
similarity) to one (identical to Earth) given some known planetary properties.
For exoplanets the ESI is based on stellar flux and either the mass or radius
of the planets. Since habitability depends on many other factors it is not known
if planets with similar mass and stellar flux to Earth (i.e., ESI values closer
to one) are also in general more habitable.

Note: The PHL @ UPR Arecibo created an independent assessment, and produced multimedia content for ESO and the general public as part of the announcement of the Proxima b discovery. We acknowledge the collaboration of Guillem Anglada-Escudé from Queen Mary University of London (lead scientist of the discovery), the ESO Public Information Office, Edgard Rivera-Valentín from the Arecibo Observatory (USRA), the computational resources of the HPCf of the University of Puerto Rico, the University of Puerto Rico at Arecibo, and the music of Lyford Rome.

Video 01. The Pale Red Dot campaign aimed to find a planet orbiting our nearest stellar neighbor, Proxima Centauri. Incredibly, the quest succeeded and the team did indeed find a planet. Even more exciting, the planet, Proxima b, falls within the habitable zone of its host star. The newly discovered Proxima b is by far the closest potential abode for alien life. Credit: ESO.

Video 02. This is an artistic interpretation of the potentially habitable planet around our nearest star, Proxima Centauri. The planet is represented here as a mostly desert-like, tidally-locked world with shallow oceans and a strong atmospheric circulation allowing heat exchange between the light and dark hemispheres. The star and orbit are to scale, but the planet was enlarged (x30) for visibility. Video was rendered by the PHL’s SER software. Additional versions of this video are availablehere. Credit: PHL @ UPR Arecibo, ESO. Background music ‘Atmospherics Final’ by Lyford Rome.

Video 03. This is a one-minute real-time simulation showing a close encounter with Proxima b at 20% the speed of light. The StarShot Initiative is planning a mission to the Alpha Centauri stellar systems at such speed. The same animation would take over two days to complete at the speed of NASA's New Horizons (~16 km/s). Video was rendered by the PHL’s SER software. Additional versions of this video are availablehere. Credit: PHL @ UPR Arecibo, ESO. Background music ‘Into the Black’ by Lyford Rome.

Video 04. This is a simulation of the possible surface temperatures of a tidally-locked Proxima b, always giving the same face to its star. The simulations show the large temperature differences between the permanent daylight and nightside hemispheres. This assumes that an ocean and atmosphere transfers heat effectively around the planet, but we do not know yet if this is the case. The temperature range includes habitable conditions, even for complex life (0-50°C). However, Proxima b may also be exposed to high UV and X-ray fluxes that could challenge any presence of life. Proxima b seems to be an extreme, but very interesting planet by terrestrial standards. Additional versions of this video are available here. Credit: M. Turbet/I. Ribas/ESO.

Images

Figure 01. Summary of the properties of the Proxima Centauri System. The size of the red-dwarf star Proxima Centauri and its planet Proxima b are approximately to scale in this diagram. The planet is at a distance of almost 20 times the Earth-Moon distance (74 star radii) from its star. For comparison, Earth is at 407 times the Earth-moon distance from the Sun. Credit: PHL @ UPR Arecibo.

Figure 03. Size comparison of the red-dwarf star Proxima Centauri and its planet Proxima b with some Solar System bodies, including Earth, Jupiter, Saturn, and the Sun. The color of Proxima Centauri and the Sun were enhanced. Credit: PHL @ UPR Arecibo.

Figure 04. The Alpha Centauri family is composed of three stars. The G-star Alpha Centauri A and its K-star companion B orbit each other in a very eccentric orbit separated from 11 to 36 astronomical units (AU). Proxima Centauri is believed to be also bound to this system, but at a distance of 15,000 AU. Our G-star Sun is shown for scale. The color of the stars were adjusted to approximately imitate the human eye perception. Credit: PHL @ UPR Arecibo.

Figure 05. Simulated comparison of a sunset on Earth and Proxima b. The red-dwarf star Proxima Centauri appears almost three times bigger than the Sun in a redder and darker sky. Red-dwarf stars appear bigger in the sky than sun-like stars, even though they are smaller. This is because they are cooler and the planets have to be closer to them to maintain temperate conditions. The original photo of the beach was taken at Playa Puerto Nuevo in Vega Baja, Puerto Rico. Credit: PHL @ UPR Arecibo.

Figure 06. Simulated comparison of a sunset on Earth with that of four known potentially habitable worlds. The sunset from the planets with red-dwarf stars (Proxima b, Gliese 667C c, and Wolf 1061 c) appear darker, but with a bigger star than those with K-stars (Kepler-442 b) or sun-like stars. Red-dwarf stars appear bigger in the sky than sun-like stars, even though they are smaller, because they are cooler and the planets have to be closer to them to maintain temperate conditions. The original photo of the beach was taken at Playa Puerto Nuevo in Vega Baja, Puerto Rico. Credit: PHL @ UPR Arecibo.

Figure 07. Comparison of the visual appearance of Earth illuminated by the Sun (left) and a red-dwarf star (right). The light from a red-dwarf star, such as Proxima Centauri, makes Earth look darker and with a pale green-yellow tone instead of the familiar pale blue. Credit: PHL @ UPR Arecibo, NASA EPIC Team.

Figure 08. Artistic representations of the top 10 potentially habitable worlds in the Habitable Exoplanets Catalog, now including Proxima b. They are sorted in this image by the Earth Similarity Index (ESI), a measure of how similar a planet is in size (given by radius or mass) and stellar flux (insolation) to Earth. Planetary habitability depends on many factors and it is not known if planets with a similar size and insolation as Earth are generally habitable. Earth, Mars, Jupiter, and Neptune for scale. Planet candidates indicated with asterisks. Credit: PHL @ UPR Arecibo.

Figure 09. Artistic representations of the top 10 potentially habitable worlds in the Habitable Exoplanets Catalog, now including Proxima b. They are sorted in this image by distance from Earth in light years. Earth, Mars, Jupiter, and Neptune for scale. Planet candidates indicated with asterisks. Credit: PHL @ UPR Arecibo.

Figure 10. Orbit of Proxima b assuming a maximum eccentricity of 0.350, but its actual orbit might be less eccentric (closer to the red dotted circle). The size of the habitable zone is shown in a green shade and the ice-line with a blue dotted circle. The orbit is well within the tidal-lock radius (outside of the frame). Credit: PHL @ UPR Arecibo.

Figure 11. This figure shows all planets near the habitable zone, now including Proxima b (darker green shade is the conservative habitable zone and the lighter green shade is the optimistic habitable zone). Only those planets less than 10 Earth masses or 2.5 Earth radii are labeled. Some are still unconfirmed (* = unconfirmed). Size of the circles corresponds to the radius of the planets (estimated from a mass-radius relationship when not available). Credit: PHL @ UPR Arecibo.

Figure 12. Sky map with all the stars with known potentially habitable planets (yellow circles). The star Proxima Centauri is close to the center bottom of the figure. Click image to enlarge. Credit: PHL @ UPR Arecibo, Jim Cornmell.